<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>边框背景</title>
    <style type="text/css">
        #box1{
            width: 10px;
            height: 30px;
            background-color: darkgrey;
            border-radius: 50px 0 0 50px;
        }
        #box3{
            width: 10px;
            height: 30px;
            background-color: darkgrey;
            border-radius: 0 50px 50px 0;
            float: right;
            margin-top: -30px;
            margin-right: 1215px;
        }
        #box2{
            float: left;
            margin-top: -28px;
            margin-left: 35px;
        }
    </style>
</head>
<body>
  <div id="box1">

  </div>
  <div id="box2">
      边框背景
  </div>
  <div id="box3">

  </div>
</body>
</html>